9. Solar eclipses

A solar eclipse takes place when the Moon partially or completely covers the face of the Sun. The Moon’s orbit around the Earth is inclined to the orbit of the Earth around the Sun. This means twice a year the Moon will appear to cross over the path of the Sun at points called the “nodes”. If the Moon crosses over the face of the Sun while the Sun and Moon are at these points, the result is a solar eclipse. However, the Sun is vastly bigger than the Moon so how can it be completely covered by it? The answer is a remarkable coincidence: while the Sun is bigger (by 400 times) it is also 400 times farther away from us than the Moon. The result is that at certain times the much smaller Moon appears to fit neatly over the face of the Sun. When this happens, we can glimpse for a few precious minutes the outer regions of the Sun often collectively referred to as the “Solar Atmosphere”.

To see a solar eclipse we must be in the right geographical location, somewhere along the “eclipse path”. This is very important for a total solar eclipse. If we are positioned away from the eclipse path we will only see a partial eclipse or no eclipse at all. Some people travel regularly overseas to see solar eclipses, particularly those that are total.

There are several types of solar eclipse:

PARTIAL: These types of solar eclipse are where the Moon only covers part of the Sun. The Moon may only cover a percentage of the solar disk at maximum eclipse. We never see the solar corona as the solar disk is always visible. A telescope or telephoto lens with a full aperture white-light solar filter are required at all times (or a safe solar eclipse viewer if you are watching the eclipse with the naked eye) during the eclipse to avoid eye damage (an "eclipse burn").

TOTAL: A total solar eclipse is where the Moon eventually covers all of the Sun's disk after going through the partial phases first. Once the solar disk is covered we have "totality". We can temporarily remove any solar filters. The delicate-white streamers of the solar corona are then clearly visible around the black circle of the Moon (though in images it is possible to just see the darkened lunar surface). This image only gives a flavour of what is visible to the named eye and through a telescope during the period of totality. Red prominences can also be seen without the aid of a hydrogen-alpha filter. The maximum duration of totality can at most be just over seven minutes long (it is often much less as each eclipse duration of of totality varies). Once the total phase is over we must again replace the solar filter to follow the partial phases as the Moon moves away from the Sun.

The "Diamond Ring" appears during a total solar eclipse. As the Moon at last covers the Sun's disk, the Diamond Ring appears briefly just before totality begins. More dramatically, the Diamond Ring reappears as totality ends as the Moon slowly allows the first sliver of the solar disk to burst out from behind the dark body of the Moon. Visually, it is brilliant especially after the subdued light of totality. You get an indication it is about to happen as the corona starts to fade from view but it is always something of a surprise when the Diamond Ring bursts forth! It also means once totality is over that we must put the full aperture solar filter back over the front of the telescope or telephoto lens or eye damage will occur.

ANNULAR: The last type of solar eclipse is the annular solar eclipse. This is where the Moon does not completely cover the Sun. The Moon's orbit around the Earth is slightly elliptical meaning the Moon's disk varies in size throughout its orbit around the Earth. We normally do not notice this happening, but if the Moon is slightly farther away from the Earth and a total solar eclipse is due, the result is that a ring of solar disk remains around the body of the Moon during totality. There will be no solar corona visible because all of the Sun's disk must be covered for it to become visible. A full aperture white-light solar filter must be used at all times to protect the eyes because the Sun's disk remains visible throughout the eclipse.

The last total solar eclipse visible from the UK occurred in August 1999 and was visible from Cornwall. The next total solar eclipse that will be visible near the UK will take place on 12 August 2026, which will be visible from Spain and at sea off the west coast of Ireland, where you would see totality of 2 minutes and 6 seconds. From mainland UK we will see only a partial solar eclipse.